The primary objective of this work is to develop a hip prosthesis with a much longer lifetime than those presently available through the substitution of polyoxymethylene for ultra high molecular weight polyethylene. Another objective, perhaps of greater urgency, is to find a material that will solve the problems of the tibial plateau of the knee joint (scarring, deformation and gross wear). It is believed that the higher creep resistance and hardness of polyoxymethylene shows promise as a material to alleviate these problems. Results of our research to date have shown that: 1) polyoxymethylene (Delrin 150) possesses superior wear resistance to ultra high molecular weight polythylene (UHMWPE) in blood plasma but not in distilled water. 2) UHMWPE possesses superior wear resistance to Delrin 150 in distilled water. 3) Normal synovial fluid within human joints contains a protein lubricating fraction which does not lie within the hyaluronate fraction and which is independent of the hyaluronate concentration. In addition to continuing wear tests in distilled water and blood plasma as well as continuing in vivo biocompatibility tests of the solid Delrin, it remains to determine the biocompatibility of wear debris that would be derived from the hip joint. Current research concerns the development of a simple method and test protocol for producing contamination-free wear debris in quantities sufficient for biocompatibility tests. Both polyoxymethylene and UHMWPE will be investigated for biological tolerance of both the bulk material and wear debris. Components will then be fabricated from the Delrin 150 for total hip prostheses and tested on a hip joint point simulator. Successful conclusion of these tests will allow clinical trials to commence.